The invention is directed to the production of pressed objects (slugs) from pyrogenically produced oxides of metals and/or metalloids.
These oxides are of high purity and extremely finely divided. The average primary particle sizes (DIN 53206) (German Industrial Standard 53206) as a rule is between 7 and 40 millimicrons. The high purity, the lack of pores and the substantially chemical inertness have made the pyrogenically produced oxides interesting as carriers for catalysts. Examples of this use have long been known in the scientific literature (see the review of Koth et al. in Chem. Ing. Techn. Vol. 52 (1980) pages 628-634). However, in all cases the pyrogenically produced oxide was employed in powder form.
Previously it was not possible to produce pressed objects from highly dispersed pyrogenic oxides, which because of the inner surface area and their mechanical strength would be suited as catalyst carriers.
To be sure there is known from German OS No. 2625705 a process for the production of a pellet shaped catalyst carrier which is characterized by heating a shaped product made from a mixture of finely divided silicon dioxide powder and colloidal silicon dioxide for the purpose of hardening. In regard to the silicon dioxide powder employed, however, there is used a silica whose particle size is from 0.5-100 micrometers.
Experiments using this process on pyrogenically produced metal or metalloid oxides having a particle size of 7-40 millimicrons are described in Koth, Chem. Ing. Techn. Vol. 52 (1980) at pages 633-634.
However, this process has the disadvantage that, e.g. from a pyrogenic silica (Aerosil 200) there can only be prepared pellets having a maximum breaking strength of 4.3 kp. An additional disadvantage is that compared to the starting material there must be accepted a reduction of the accessible surface area of somewhat more than 25%. Pressed objects of pyrogenically produced aluminum oxide (Al.sub.2 O.sub.3 -C) are likewise described in the mentioned Koth article. They have a breaking strength of 5.6 kp.
The production of pressed objects from pyrogenically produced oxides accordingly appears to be possible only by making allowance for the low breaking strength and the great decrease of the surface area compared to the starting material.
The object of the invention is to form pressed objects (e.g. slugs, pellets or briquettes) from pyrogenically produced oxides which have a high breaking strength and a surface area which is only a little below that of the starting material.